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Old 26-10-2009, 20:29   #1
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Planing Hulls with Twin Diesels...Economical?

So, I've been looking for boats...feeling like kind of a traitor for looking at powerboats but I'd like to live aboard and they just provide a lot more amenities. I don't really have enough money to support a total gas hog though. OTOH, I live a decent ways away from most places I'd like to go and being able to do 10-20 knots would greatly expand my weekend destinations.

I've seen a few boats on craigslist recently with twin 110-175 hp engines, from 28' to 40'. I'm just kind of curious as to how well these boats work. The diesels seem to be about half the HP of similar boats with gas engines yet they are claiming to have about the same cruising speeds.

Are they a lot more economical than their gas counterparts?

Some examples of ads for faster cruisers:

TWIN DIESEL 20 KNOT CRUISE LOW HOURS

32' Bayliner Conquest, 1984, Twin Volvo Diesel Engines! Meticulous!

And slower more trawler-like ones:

BAYLINER MOTORYACHT 40'

1986 Bayliner 3270 Twin Diesel
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Old 26-10-2009, 22:13   #2
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So, I've been looking for boats...feeling like kind of a traitor for looking at powerboats but I'd like to live aboard and they just provide a lot more amenities.
Go displacement, 50 foot trawlers with 100hp^v motors can get around 1 litre/NM doing 8 knots
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Old 26-10-2009, 22:21   #3
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My current budget is limited to about $40-50k max so 50' trawlers are basically out of the picture. And if I could afford a 50' trawler I'm guessing the expense of gas wouldn't be as big of a deal...
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Old 26-10-2009, 23:06   #4
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Diesel is definitely more efficient. You get roughly 30% more energy from a gallon of diesel. As for planing versus displacement, displacement is clearly more efficient by a huge margin.

Also, I would steer clear of Bayliners in general. There are many other better built used powerboats out there, especially in this economy. For planing powerboats, you may want to have a look at used SeaRays. Cummins and Caterpillars tend to be good diesel engines in that horsepower range. The newer John Deere's are also good. Avoid Volvo's...the parts extremely expensive.
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Old 26-10-2009, 23:11   #5
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My current budget is limited to about $40-50k max so 50' trawlers are basically out of the picture. And if I could afford a 50' trawler I'm guessing the expense of gas wouldn't be as big of a deal...
A mate of mine had a 45 ft one of these that he got for $47k
This one uses 6 lph to do 8 knots
Cray Boat 40ft

Another mate of mine got one like this a few years back for around the same money as above without all the trawl gear on it TRAWLER 53 but in immaculate condition

A quick google shows this in the states 1979 Mainship 34'
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Old 26-10-2009, 23:34   #6
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Yeah, there are a few mainship 34's, some CHB 34's, a Hershine 37' in Nanaimo, and a few misc. somewhat custom trawlers in that price range.

I agree that bayliners are not exactly quality boats.

I know displacement hulls with appropriately sized engines are more efficient by a long shot.

Just curious mainly in how diesel planing boats compare to gasser planing boats. For example, in the case of the Bayliner Conquest, it has 2x165hp diesels and claims a cruising speed of 20 knots. The gasser versions seem to have 2x260hp gas engines and about 20-22 knots cruising. So, I'm wondering how much more efficient diesels would be. I imagine the gas engines on that boat get around 1.5mpg at cruise.
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Old 27-10-2009, 10:55   #7
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I agree that bayliners are not exactly quality boats.
The problem there is that this statement almost always comes from people that have never owned one.

(/I've never owned one)

Judge it on it's own merits. Not the word of others that may not have firsthand knowledge.
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Old 27-10-2009, 12:40   #8
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My parents had a Bayliner 30 when I was in high school and I spent a few months on it. I can't remember the year. It's not like they're going to spontaneously sink or anything but they just aren't built very well. The stanchions and railings had a lot of give to them, there was spider webs of cracks in the fiberglass, etc. Other bayliners or bayliners from different years may be built better. Some of them can be a lot of boat for the money though and I don't intend to take one out on the ocean, so I'm not totally writing them off. And it's not like they're the only poorly made powerboats.

I'm still trying to figure out how you're supposed to dock these big powerboats with no deck doors though...just seems dangerous as hell with all that windage and no keel/ballast. Put her in neutral and let her steer herself intot eh slip while you run 20 feet to the cockpit to grab the lines?
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Old 27-10-2009, 12:49   #9
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Yea... I've seen Carvers that made me scratch my head and ask the same question.

"So you have to go from the bow, run across the deck, up the stairs, down either the cockpit stairs or down the ladder, then open the surround glass to grab the stern line?... Ummm... Good luck with that on a rainy day Mrs. First Mate." :-)
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Old 27-10-2009, 12:56   #10
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Yeah, there are a few mainship 34's, some CHB 34's, a Hershine 37' in Nanaimo, and a few misc. somewhat custom trawlers in that price range.

I agree that bayliners are not exactly quality boats.

I know displacement hulls with appropriately sized engines are more efficient by a long shot.

Just curious mainly in how diesel planing boats compare to gasser planing boats. For example, in the case of the Bayliner Conquest, it has 2x165hp diesels and claims a cruising speed of 20 knots. The gasser versions seem to have 2x260hp gas engines and about 20-22 knots cruising. So, I'm wondering how much more efficient diesels would be. I imagine the gas engines on that boat get around 1.5mpg at cruise.
For the same hull, a diesel is 30% more efficient minus the additional inefficiencie caused by diesels being heavier engines for a given horsepower. For a planing vessel, weight is a more critical factor than for a displacement vessel.

Your miles per gallon figure (or gallons per mile) is going to vary with the hulls displacement, hull shape and how fast you want to go which is a cubed relationship with maximum horsepower. To double the speed you pretty much must cube the horsepower.

You can look up an engines gallons per hour figure at a given horsepower or RPM by going to the engine manufacturers website.
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Old 27-10-2009, 13:34   #11
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Yea... I've seen Carvers that made me scratch my head and ask the same question.

"So you have to go from the bow, run across the deck, up the stairs, down either the cockpit stairs or down the ladder, then open the surround glass to grab the stern line?... Ummm... Good luck with that on a rainy day Mrs. First Mate." :-)
The carver aft cabins in particular seem like a disaster...but there must be some way...seems like you would have to run back out the stairs to the sundeck, grab the lines, then forward to the side decks, then a bounding leap off the high sides....

From what I'm reading it sounds like the higher repair and replacement costs for the diesels outweigh the fuel savings.
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Old 27-10-2009, 14:09   #12
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The carver aft cabins in particular seem like a disaster...but there must be some way...seems like you would have to run back out the stairs to the sundeck, grab the lines, then forward to the side decks, then a bounding leap off the high sides....

From what I'm reading it sounds like the higher repair and replacement costs for the diesels outweigh the fuel savings.
True, you have to burn a lot of diesel for them to pay for themselves. Gas engines are more economical for periodic use.
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Old 27-10-2009, 14:10   #13
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In the early 80's I used a 34' (36 with bowsprit) Allmand sedan cruiser with twin renault 160 HP diesels. Wonderful boat, but couldn't get parts for the engines. The boat burned 1.5 gph at 6 mph and 6 gph at 23 mph cruise. Either way it got 4 mpg. The boat would do 40 mph but sucked fuel at that speed. It carried 350 gallons of fuel so could really go a long way. The hull never pounded even at cruise speed in 6 foot seas. These twin diesels can indeed be very economical.
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Old 31-10-2009, 13:38   #14
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Diesel VS gas

When you need more HP at cruise- go diesel.
When you plan to run lots of hours- go diesel.
There is the safety factor too. go diesel

No matter what fuel you have, weight is the biggest factor in efficiency.
You plan to live aboard, so there is NO WAY that you will be the first human to live light enough to have a 30Kt boat that gets good mileage.

For an example of a light fast efficient boat, compare to the MJMz boats.
I said compare cause they are $$$$.

For the most boat for the $$ you are stuck with a slow boat. Think about it though, 8Kts into the wind is still way better than any sailboat.
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Old 06-11-2009, 13:05   #15
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You want to live aboard, therefore you will probably have lots and lots of stuff. While one can have a very fuel efficient planing boat (7mpg, 10mpg), such boats are VERY light: think a couple of thousand pounds, probable far less than your personal gear weighs, to say nothing of equipment (galley, head, electrical)...

Except for these extreme (and in your case unsuitable) specimens, pretty much any used powerboat that goes over 8 knots and costs less than $250K will consume fuel at the rate of 1 gallon per mile, and usually, much more. This even applies to "fast trawlers" like Nordic Tugs and similar.

The MJZ and a very few other boats achieve a bit over 2nmpg but they are using state-of-the-art powerplants that are VERY expensive to buy and maintain. But I am sure that nearly everyone (this side of the Pardy's) will find such boats MUCH too small to live aboard, or even stay aboard for more than a weekend. And they cost an order of magnitude more than your budget, so its not worth talking more about these exceptional examples.

You really should restrict your search to non-planing boats, and the longer the better. As Highlander said, 8 knots will get you places a lot faster than you are used to sailing.

On displacement boats, the slower you go, the more efficient.
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